This invention relates in general to optical switches and, more particularly, to electrooptic waveguide switches for use with multimode optical fibers.
Present integrated-optics switches and modulators are, in general, single-mode devices which are suitable for use only with single-mode optical fiber transmission lines. The fiber-to-film coupling loss is a serious obstacle to the practical utilization of single-mode fiber-switch systems. A primary source of this coupling loss is misalignment between the fiber and the film waveguide. The alignment problem arises because single-mode fibers are typically 2-4 microns in core diameter while the present techniques for alignment are in general accurate to within 1-2 microns. Substantial loss can result from a very small misalignment and, thus, alignment tolerances are very demanding. However, alignment tolerances in a multimode system are much less demanding. Since multimode fibers typically have a core diameter of 50-80 microns and the channel waveguides of the multimode devices have a cross-sectional area of corresponding size, the misalignment losses can be made acceptably small with present alignment techniques. In particular, end-fire coupling techniques may be used to couple multimode fibers to multimode channel waveguides.
Although there are many designs of single-mode switches and modulators suitable for single-mode communication systems, there is a need for active devices suitable for multimode communication systems. A. R. Nelson et al., Applied Physics Letters, Vol. 28 No. 6, p. 321, and R. A. Soref et al. Applied Physics Letters, Vol. 28, No. 12, p. 716, describe a multimode modulator and a multimode three-port switch, respectively, which electrooptically induce channels in a thin slab of LiNbO.sub.3. The present invention is an improvement on this previous work and provides a four-port active electrooptic switch which is suitable for use with large-diameter-core multimode fibers.